Recently, with remarkable development of an information communication technology, the number of users who make use of a wireless communication is now rapidly increasing. However, despite much popularity of the wireless communication, the wireless communication is seriously exposed to inevitable attacks in a wireless environment.
That is, information leakage in the information communication technology due to an authorized information exposure or operation is explosively increasing demands for a security mechanism such as cryptography and the like. Particularly, in the multi-channel age of digital broadcasting using satellites, subscribers thereof can receive an individual specialized channel service. Whereas broadcasting providers allow only a person who pays a reasonable receiving fee to watch the TV program by applying the subscriber concept to the TV broadcasting in the broadcasting service management depending on only advertisement rates in existing terrestrial TV broadcasting, and also allow specialized broadcasting providers to produce specialized broadcasting programs, so that they come to be able to provide services of various functions.
A system which can satisfy such a conditional access service corresponds to a conditional access system (CAS). The conditional access system enables only a subscriber of a receiving side, who is authorized to receive, to descramble a signal scrambled by a transmitter and to watch the TV programs.
However, whenever there are service requests from the client, since such a conditional access system encrypts a group key by means of a private key of a client for the purpose of the group key update by a server, and performs a unicast transmission of the group key to all of the clients, unnecessary delay occurs.